Open railgun with steel barrel sections

ABSTRACT

An elongated electromagnetic railgun ( 1 ) adapted to propel a moving armature ( 30 ) through a bore ( 11 ) along the length of the railgun ( 1 ) from its breech end ( 21 ) to its muzzle end ( 22 ). The railgun ( 1 ) comprises two elongated mechanically rigid electrically conductive barrel sections ( 13 ), said sections ( 13 ) being spaced apart from each other along the length of the railgun ( 1 ). Mechanically coupled via a dielectric ( 18 ) to each barrel section ( 13 ) is an elongated current carrying rail ( 14 ) for providing electromagnetic propulsive force to the armature ( 30 ). The two rails ( 14 ) face each other across an elongated open channel, defining the bore ( 11 ). The two barrel sections ( 13 ) are electrically connected to each other at a maximum of one location of the railgun ( 1 ).

RELATED APPLICATIONS

This patent application is a continuation-in-part of commonly owned U.S.patent application Ser. No. 13/426,399 filed Mar. 21, 2012, which claimsthe benefit of commonly owned U.S. provisional patent applications61/475,414 filed Apr. 14, 2011; 61/488,614 filed May 20, 2011;61/513,729 filed Aug. 1, 2011; 61/525,303 filed Aug. 19, 2011;61/549,928 filed Oct. 21, 2011; 61/567,070 filed Dec. 5, 2011;61/588,498 filed Jan. 19, 2012; and 61/595,110 filed Feb. 5, 2012; andalso claims benefit of commonly owned international patent applicationPCT/US2013/022032 filed Jan. 18, 2013; all ten of which previously-filedpatent applications are hereby incorporated by reference in theirentireties into the present patent application.

TECHNICAL FIELD

This patent application pertains generally to the field ofelectromagnetic launchers, and specifically to railguns.

BACKGROUND ART

Background references include the following references, all of which arehereby incorporated in their entireties into the present patentapplication:

-   -   1. “Electrical and Thermal Modeling of Railguns”, Kerrisk, Jerry        F., IEEE Transactions on Magnetics, Vol. Mag-20, No. 2, March        1984, pp. 399-402, U.S.A.    -   2. “Loss of Propulsive Force in Railguns with Laminated        Containment”, Parker, Jerald V., and Levinson, Scott, IEEE        Transactions on Magnetics, Vol. 35, No. 1, January 1999, U.S.A.    -   3. “Eddy Current Effects in the Laminated Containment Structure        of Railguns”, Landen, Dwight and Satapathy, Sikhanda, IEEE        Transactions on Magnetics, Vol. 43, No. 1, January 2007, U.S.A.    -   4. “Phenomenological Electromagnetic Modeling of        Laminated-Containment Launchers”, Mallick, John, IEEE        Transactions on Magnetics, Vol. 43, No. 1, January 2007, U.S.A.

5. “Enhancement of the Compressive Strength of Kevlar-29/Epoxy ResinUnidirectional Composites”, D'Aloia et. al, High Performance Polymers,Vol. 20, pp. 357-364, June 2008, first published Dec. 11, 2007.

-   -   6. Quickfield Version 5.7, Finite Analysis System, Tera        Analysis, Ltd., Svendborg, Denmark, 2009, http://quickfield.com        (last downloaded Nov. 1, 2010)

Kerrisk [Reference 1] taught that a gun barrel electrically conductivealong the major gun axis could not be brought into close proximity tothe current carrying rails of a railgun without significantly reducingrail inductance. Given the barrel geometry, which was fully enclosing ofthe rails, and the other boundary conditions used, the conclusionsarrived at were correct.

However, consider the following. The gas law is represented by a scalarequation and hot gas produces an isotropic pressure. Consequentially,the barrel for a standard gun must be everywhere continuous in theta andz to prevent gas escape and force loss on the back projectile surface.On the other hand, the magnetic field is defined by Maxwell's equations,and the magnetic field is a vector quantity. It follows that themagnetic pressure is a vector quantity. The barrel design for a magneticgun can take advantage of this fundamental difference between these twocases. It is not necessarily required that the barrel be continuous intheta and z for full magnetic pressure containment and for the magneticpressure to be properly applied to the back armature surface. That is,the barrel need not be fully enclosing of the rails.

If the electrically conducting gun barrel: (1) is split open top andbottom from the breech to the muzzle, and (2) the two new barrelsections make contact with each other only at the gun base (i.e., thegun breech), the condition for completing the image current circuit inthe armature region can no longer occur, as discussed by Kerrisk[Reference 1]. This represents the case where each of the twoindependent barrel sections is mechanically anchored to the gun basewith direct metal-to-metal mechanical contact. Therefore, the barrelsections are electrically connected to each other at the base. However,the two barrel sections remain electrically isolated from each othereverywhere else along the length of the gun barrel. This new barrelconfiguration is described herein.

DISCLOSURE OF INVENTION

An elongated electromagnetic railgun (1) adapted to propel a movingarmature (30) through a bore (11) along the length of the railgun (1)from its breech end (21) to its muzzle end (22). The railgun (1)comprises two elongated mechanically rigid electrically conductivebarrel sections (13), said sections (13) being spaced apart from eachother along the length of the railgun (1). Mechanically coupled via adielectric (18) to each barrel section (13) is an elongated currentcarrying rail (14) for providing electromagnetic propulsive force to thearmature (30). The two rails (14) face each other across an elongatedopen channel, defining the bore (11). The two barrel sections (13) areelectrically connected to each other at a maximum of one location of therailgun (1).

BRIEF DESCRIPTION OF THE DRAWINGS

These and other more detailed and specific objects and features of thepresent invention are more fully disclosed in the followingspecification, reference being had to the accompanying drawings, inwhich:

FIG. 1 is an exploded isometric view of one embodiment of railgun 1 ofthe present invention.

FIG. 2 is a cross-sectional view of the FIG. 1 embodiment.

FIG. 3 is an isometric view of the embodiment of FIG. 1 in whichretention frames 32 are used.

FIG. 4 is an isometric view of a retention frame 32.

FIG. 5 is a magnetic field line plot for the embodiment of FIG. 1.

FIG. 6 is a closeup of a portion of the magnetic field line plot of FIG.5.

FIG. 7 is an isometric view of the FIG. 1 embodiment showing a firstmeans for electrically coupling the barrel sections 13 to each other atthe base 23.

FIG. 8 is an isometric view of the FIG. 1 embodiment showing a secondmeans for electrically coupling the barrel sections 13 together at thebase 23.

FIG. 9 is an isometric view of the embodiment of FIG. 8 in which adielectric shell 10 has been added.

FIG. 10 is a modification of the FIG. 1 embodiment in which the barrelsections 13 are tapered, and the cross-section of each barrel section 13is a non-square rectangle.

FIG. 11 is a top view of the FIG. 10 embodiment.

FIG. 12 is an isometric view of a second embodiment of the railgun 1 ofthe present invention.

FIG. 13 is a cross-sectional view of the barrel section 13 of FIG. 12.

FIG. 14 is an isometric view of the FIG. 12 embodiment showing the useof retention frames 32.

FIG. 15 is a magnetic field line plot for the FIG. 12 embodiment.

FIG. 16 is a closeup of a portion of the magnetic field line plot ofFIG. 15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

By electrically isolating two specially designed electrically conductivegun barrel sections 13 along their lengths from breech 21 to muzzle 22,the present invention changes the boundary conditions used in the 1984paper by Kerrisk [Reference 1]. The result allows a metal gun barrel 13to be located close to the current carrying rails 14 while stillmaintaining high inductance per unit length (L′). The following is adescription of a two-rail 14 open air railgun 1 that uses this principleto achieve high efficiency. Each current carrying rail 14 ismechanically supported by its own barrel section 13. The rails 14 arenormally identical to each other, and are spaced apart from each otheralong the entire length of the railgun 1. The space between the rails 14defines the gun bore 11. The bore 11 is directly exposed to theatmosphere (ambient gases) along its entire length.

First Principal Embodiment

FIG. 1 shows an isometric view of a first principal embodiment of thepresent invention. The spacing between the rails 14 is typically 30 cmfor the particular design used to calculate results discussed here,including the inductance (L′=0.40 uH/m) and the magnetic computationaloutputs presented below. The results presented are not optimized at thesystem level. For example, with all else being held constant, if the gunbore 11 is increased from 30 cm to 40 cm, L′ increases from 0.40 uH/m to0.47 uH/m.

In this two-rail 14 system, current is carried along the first rail 14,conducted across a moving armature 30 (see FIG. 2), and then returned inthe opposite direction along the second, parallel, and opposing rail 14.

Image currents create the opportunity for a two-part rail 14,6. Thefirst part 6, where sliding contact between the rail 6 and the armature30 is made, is made of steel or a similar highly wear-resistantmaterial. The second part 14, which makes up the bulk of the rail 14,6and which carries the bulk of the current to and from the generator, ismade of copper or copper alloy. This design allows for a large enoughrail 14,6 size to both accommodate rail 14,6 cooling and a reducedresistance per unit length, to more than counter the increased powerloading due to the introduction of image currents on the outer surfacesof barrel sections 13.

Circular rail part 14 is preferably recessed within its correspondingbarrel section 13. While other materials could be used for plate 6,steel is usually the material of choice, even though the thermalexpansion coefficients of steel and copper alloys are quite different.Other materials, such as tungsten alloy, tungsten copper eutectic, or atungsten copper alloy, could be used for plate 6. This would bettermatch the CTE's of the two parts 14,6.

Plate 6 is explosion bonded, or otherwise firmly attached, to the copperalloy rail part 14. Each plate 6 preferably has small periodicallyspaced slots 7. The slots 7 are perpendicular to the long (z) axis ofthe rail 14,6. Slots 7 extend all the way through plate 6. In this way,as the copper 14 expands and contracts at a greater rate than the steel6, the small sections of steel 6 can absorb the small differentialstresses and strains that occur as the temperature cycles between eachrailgun 1 shot.

In an alternate embodiment, the two parts 14,6 of the rail can bereplaced with a single part 14 made entirely out of a single material,such as tungsten copper alloy.

The copper or copper alloy part 14 contains a large continuous orsectional channel 5 interior to the rail part 14. Channel 5 allows forthe flow of coolant, either along the entire rail 14,6 length, or,preferably, along a plurality of rail 14,6 sections. In this embodiment,holes 17 can be machined into the barrel sections 13, and coolant can beexchanged at varying gun 1 locations. The total heat deposited variesalong the rail 14,6. Therefore, rail 14,6 cooling can be better managedin sections, as provided by this embodiment.

While not shown, the gun barrel 13 can be cooled independently of therails 14,6.

The barrel sections 13 provide structural integrity to the railgun 1. Inconjunction with the retention frames 32, sections 13 contain the strongoutward lateral forces that are produced by the magnetic pressure fromthe (typically very high) currents flowing through the rails 14,6.

Typically, the two barrel sections 13 are electrically connected to eachother at just one location along the length of the gun 1, namely, at theregion of the base 23, as shown in FIGS. 7, 8, and 9. However, in someembodiments, the two barrel sections 13 are not electrically connectedto each other or to any other electrically conductive mass (e.g., base23, turret 20, or deck 4) at all, i.e., they are made to “float”electrically. This “electrically floating” design can be used for allthe embodiments illustrated herein, i.e., those depicted in FIGS. 1through 6, as well as FIGS. 10 through 16.

Barrel sections 13 are typically made of a high-strength metal, such assteel. The outer surfaces of the barrel sections 13 may be lined withelectrically conductive linings 16, so that these outer surfaces areelectrically conductive to a higher degree. This facilitates the returnof image currents from the muzzle end 22 to the breech end 21 with lowerlosses. When used, linings 16 are fabricated of a very highlyelectrically conductive material, such as copper or copper alloy.

The two barrel sections 13 are spaced apart from each other, at auniform distance, throughout the length of the railgun, and are normallyidentical to each other.

FIG. 2 is a cross-sectional view of a barrel section 13 and its embeddedcurrent carrying rail part 14. In this particular embodiment, barrelsection 13 has a square or non-square but rectangular cross-section.Typical dimensions are 55 cm×52 cm for the cross-section of barrelsection 13, 30 cm diameter for rail part 14, and 2.5 cm thickness forinsulator 18. The gun bore 11 is represented by the volume between thetwo facing steel plates 6. FIG. 2 illustrates the left-hand plate 6. Thebore 11 volume is entirely open to the atmosphere (ambient gases) fromthe breech 21 to the muzzle 22, except when armature 30 and anyaccompanying projectile 31 pass through the bore 11. Armature 30 canitself be the payload, or it can propel a separate projectile 31 whichconstitutes the payload.

In many embodiments, such as illustrated in FIG. 2, the barrel 13surfaces are lined with copper or copper alloy lining 16. In somecircumstances, such as to save weight, aluminum or aluminum alloy mightbe used for linings 16. The region between the current carrying railparts 14 and the gun barrel 13 is mostly filled with a dielectric 18.Kevlar is the dielectric 18 of choice, although in other applications,Phenolic, ceramic, or a ceramic composite can be used. The geometry isdesigned to insure that there is no direct line of sight between thedielectric 18/air interface and the sliding contact region between therail 14,6 and the armature 30. The purpose of this geometricalconstraint is to prevent direct UV illumination of these surfaces. Italso prevents direct liquid metal (emanating from the armature 30/rail14,6 interface) or other direct sputtering or evaporative inducedcoating of the insulator 18 surface. Additional baffles can be added tofurther protect this dielectric 18/air interface, if required.

Plate 6 is concave (from the point of view of bore 11). This allows foran effective mechanical capture and guidance of the armature 30 andpayload 31 along the gun bore 11. It also helps to insure that anyliquid metal jetted from the rail 14,6/armature 30 interface will beejected directly into the outside region of the gun 1 and well away fromthe insulators 18.

FIG. 3 shows two of the retention frames 32 that hold the barrelsections 13 in place during a shot. In practice, several retentionframes 32 are used, spaced apart along the length of the gun 1. FIG. 3shows a partial assembly of the barrel sections 13 at a region otherthan the region near the base 23 and turret 20. All inward facingsurfaces of the retention frames 32 that would otherwise come in contactwith the electrically conductive barrel 13 surfaces are lined withKevlar or other suitable dielectric 35. This is to prevent theelectrical interconnection of the two barrel sections 13 with eachother. These dielectric sections 35, like dielectrics 18, are alwaysunder compression.

As shown in FIGS. 3 and 4, each frame 32 has two wedge sections 33 withsharp edges, positioned on the top and bottom of the frame 32. Wedges 33prevent the hot, high velocity liquid metal that is jetted from thearmature 30/rail 14,6 interface from coming into contact with thisportion of the retention frames 32. The sharp edges are used to preventany appreciable backsplash of the hot liquid metal. The liquid metal isreleased into the atmosphere, where it is burned off.

During the short periods that the jetted liquid metal comes into contactwith any of the retention frames 32, an alternate conducting path isproduced for current flow between the rails 14,6 other than through thearmature 30. This current path is highly resistive and highly inductivecompared to the normal path through the armature 30. Therefore,relatively little current flows along this path. The retention frames 32can be coated with a non-conductor along their beveled surfaces 33 toprevent current flow along this path.

The pitch of the retention frames 32 (distance between adjacent frames32 along the z axis) is large compared to the thickness of each frame32. This is important so as not to reduce the rail 14,6 inductanceappreciably. This also helps keep the added weight in check and ispossible for two reasons. The frame 32 height can be increased asnecessary to insure that the induced stress in the frame 32 due to therail 14,6 current-induced magnetic pressure is well within the stresslimit of the (typically steel) material from which the frames 32 arefabricated. Secondly, the barrel sections 13 are substantial in physicalsize and prevent outward deflection of the rails 14,6 in the regionsbetween retention frames 32.

Each retention frame 32 contains at least one (typically horizontal)separation slot 34, cut completely through the frame 32, located in thevicinity of a gun barrel 13 back surface 12. This prevents the completeencirclement of the rails 14,6 by a conductor which would otherwisereduce the gun 1 inductance per unit length [References 2, 3, 4].Because of the size and design of the retention frames 32, these slots34 do not compromise the frame's 32 structural integrity. The largeframe size 32 on the barrel backside 12 reduces the inductance onlymarginally, as the flux density in this region is low. The conservativecomputer modeling estimate for this flux density is approximatelybetween 0.2% and 0.3%.

The following is a calculation of the cross-sectional area of theretention frames 32 required to prevent separation of the rails 14,6 sothat the retention frames 32 do not fail. Superimposed on FIG. 5 is acontour 8 used for the computer-based net outward force calculation(1.2×10⁷ N/m) for a current of 6 MA flowing in each current carryingrail part 14. A tensile strength of 800 MPa for heat treated steel isassumed.

1 MPa=1×10² N/cm²

Therefore, the yield strength of steel (800 MPa) is:

8×10⁴ N/cm²

The cross-sectional area per unit length along the gun bore 11 axisrequired to prevent lateral rail 14,6 expansion at the yield strength ofthe material is then:

(1.2×10⁷ N/m)/(8×10⁴ N/cm²)=1.5×10² cm²/m

For a 400% engineering safety margin, this becomes:

6.0×10² cm²/m

For every meter along the gun bore 11 length, the steel cross-sectionalarea that spans the bore 11 region required with a safety factor of 400%is given by the above. Because there are two segments (one on the topand one on the bottom) to each retention frame 32 that spans the gunbore 11, each such steel cross-section is 300 cm². If each frame 32element were 30 cm in height, the retention frame 32 width would be 10cm. This represents a mechanical transparency factor of 90%. However,the magnetic transparency factor will be higher, as the flux lines areducted around the frames 32, preserving a high degree of the energydensity on both sides of the frame 32. However, and for example, theoptimized mechanical design might call for two retention frames 32 permeter, each 5 cm in width.

The 400% engineering safety margin accounts for a number of factors,including mechanical safety to failure. Of equal importance are suchfactors as magnetically induced lateral rail 14,6 displacement at thepoint of armature 30 contact. The overall mechanical system must besufficiently rigid to insure that rail 14,6 spacing and planarityspecifications are met.

The compressive strength of Kevlar is variously given in the literatureas being between 200 Mpa and 300 Mpa, and with heat treatment can be ashigh as 500 MPa [Reference 5]. The insulation 35 surface area used onthe backsides 12 of the barrel sections 13 can be designed toaccommodate similar engineering safety margins as that used above.

Magnetic Analysis

FIG. 5 shows a two-dimensional magnetic field line plot for theembodiment where the two barrel sections 13 are electrically connectedto each other at the gun base 23 only. The two barrel sections 13 remainelectrically isolated from each other everywhere else along the gunbarrel 13 length, from the breech 21 to the muzzle 22. Physically, thismeans that the two barrel sections 13 are mechanically secured to thegun base 23 with direct metal-to-metal connections. The retention frames32 were not included in the computer run that was used for FIG. 5.Inclusion of frames 32 would alter the results by approximately 2% to3%. The field line plot of FIG. 5 was taken 100 micro-seconds into thepulse. Copper was used to simulate the barrel sections 13. Thissimulated the copper linings 16 typically used around steel barrel 13walls. Due to computer program limitations, each plate 6 was simulatedusing copper fused with its corresponding copper rail part 14. There wasno accounting for transient thermal heating of the electrodes 14 thatwould otherwise drive the surface currents deeper into the electrodes 14with time. Version 5.7 of Quickfield [Reference 6] was used for allcomputational work presented herein.

A substantial amount of additional steel can be added to the gun barrelsections 13 without compromising the railgun 1 inductance. For example,in one computer simulation in which a substantial amount of steel wasadded laterally to the backside 12 of each barrel section 13, theinductance per unit length decreased from 0.40 uH/m to 0.39 uH/m. Thisis approximately a 2.5% reduction in the inductance.

FIG. 6 shows an enlargement of the space around one of the currentcarrying copper rails 14,6. The field line density has been increased toshow more precisely where the surface currents will flow after thearmature 30 has passed and before significant current diffusion into theconductors 14,6 has occurred. Drive currents flow on the surfaces ofrail 14,6, and image currents flow on surfaces of the steel barrel 13.Once the armature 30 has passed, a high percentage of the rail 14,6current is drawn into the small gap 26 between the copper rail part 14and the barrel 13. By proper design, the rail 14,6 current is initiallydistributed uniformly over the significantly enhanced surface area ofthe enlarged rail 14,6,26. Later, as the current diffuses into the bulkof the rail 14, it does so more uniformly. This minimizes energydissipation in the rail 14,6.

What is not shown accurately in FIG. 6, as Version 5.7 of Quickfield isunable to simulate such, is that due to the higher resistivity of thesteel plate 6, compared with the copper rail part 14, a higherpercentage of the rail 14,6 current on the front surface quicklymigrates to the copper part 14 than is shown in FIG. 6. Some of thesurface current immediately migrates to the copper 14 surface because ofthe relatively higher resistivity of the steel 6. Second, the highersteel 6 resistivity and ensuing heating of this material then drivesadditional surface current to the copper 14 surface, and leads to fastercurrent diffusion into the bulk steel 6 and subsequent flow in theunderlying copper part 14. Version 5.7 of Quickfield does notdynamically simulate material temperature increases and self-correct forthese related resistivity changes.

Each separatrix 27 shown in FIG. 6 denotes a location on the barrel 13surface where the surface current changes direction. These currentsreconnect at the base 23 region, where the rail 14,6 currents originate,and in the vicinity of the armature 30. Energy flow into the combinedrail 14,6 and copper lining 16 can be relatively low, as compared toexisting railgun designs. In addition, the design described in thispatent application allows for multiple firings, as the rails 14,6 areactively cooled.

The barrel 13 surface current density is highest on the surface thatfaces the copper rail part 14. It is of value that the lining 16 beparticularly thick in this region, as shown in FIG. 2. The area overwhich the image currents flow in the opposite direction is substantiallylarger, the surface current densities are lower there, and therefore thelining 16 thickness can be thinner there. See FIGS. 2 and 6. Adjustmentof the lining 16 thickness in this region can be further adjusted, basedon the specific surface current density distribution around the barrel13 circumference.

FIGS. 7, 8, and 9 illustrate different techniques for electricallycoupling the two barrel sections 13 together at the base 23. FIG. 7illustrates an electrically conductive clamping member 24, generally inthe shape of the letter “C” lying on its back, positioned at the base 23region. Clamping member 24 provides mechanical support as well aselectrical connectivity between the two barrel sections 13. In each ofFIGS. 7, 8, and 9, member 24 can be as long (in the z direction) asneeded for its mechanical purposes, although lengthening member 24causes a reduction in L′ in the regions of the base 23. Clamping member24 is supported by and electrically connected to an electricallyconductive support base 23 that rests on and is electrically connectedto an electrically conductive turret section 20. In the illustratedembodiment, turret section 20 rotates with respect to, is mounted on,and is electrically connected to, a flat electrically conductive surface4, such as the deck of a ship. The FIG. 7 embodiment illustrates therails 14,6 passing through an “open” contact area in the region of thebase 23.

In the embodiment illustrated in FIG. 8, on the other hand, electricallyconductive clamping member 24 surrounds the barrel sections 13 and rails14,6 in the region of the base 23, forming a full “closed” contact area(aperture). In other respects, FIG. 8 is identical to FIG. 7. Themagnetic field geometry in the gun bore 11 is the same for the FIGS. 7and 8 embodiments. This has been confirmed experimentally.

FIG. 9 is identical to FIG. 8, except that a dielectric shell 10 hasbeen added. Shell 10 has the same outer dimensions as clamping member24, is hollow, extends along the entire length of the railgun 1, andsurrounds all the other components in the system, including retentionframes 32. Shell 10 allows the magnetic field to escape into the regionoutside of the barrel 13 region and therefore maintain a high L′. Thisprotects the rails 14,6 from the surrounding environment. It also allowsthe rail 14,6 region to be filled with an inert gas, such as helium,nitrogen, or argon. This prevents the hot liquid metal, mostly aluminum,that is jetted from the armature 30/rail 14,6 interface from immediatelybursting into flames all along the gun bore 11 as the armature 30accelerates through the bore 11. The jetted metal can collect andsolidify on the dielectric 10 cover, which can be easily replaced asrequired.

FIG. 10 illustrates an alternative embodiment that differs from the FIG.1 embodiment in two respects: first, the cross-section of each barrelsection 13 is not a square, but rather a non-square rectangle. Thistechnique can be fruitfully used to add additional mass to thestructure, e.g., for reasons of increased mechanical support. The seconddifference in this FIG. 10 embodiment is that the barrel sections 13,when viewed from the top or the bottom, are tapered, with these sections13 being wider at the breech end 21 than at the muzzle end 22. Thepurpose of the tapering is to extend the length of the bore 11 comparedwith a non-tapered design. Lengthening the bore 11 can advantageouslycause either a higher exit velocity for the projectile 31 for a givenset of design parameters, or, alternatively, a relaxation in thesedesign parameters for a given exit velocity. This tapering technique canbe used with all of the embodiments of the present invention that aredescribed herein.

FIG. 11 is a top view of this tapered alternative embodiment, with theretention frames 32 being oriented orthogonal to the gun bore 11 axis.Note that the tapering is stepwise rather than continuous, i.e., thereis no tapering where the retention frames 32 are located. This is donefor ease of mechanical assembly and for increased mechanical strength.

Second Principal Embodiment

Because of the relatively large cross-sectional size of the conductingrails 14 in the embodiment illustrated in FIGS. 1 through 6, theinductance per unit length (L′) was limited from 0.4 uH/m to 0.47 uH/mfor a bore 11 spacing of between 30 cm and 40 cm. The current carryingrail 14 size was made large, in part to accommodate the substantial openchannel 5 interior to the rail part 14 used to flow cooling fluidthrough the rails 14,6 (or sections thereof). It is primarily throughthe reduction in the cross-sectional size of the rail part 14 thatfurther increases in L′ can be attained. It is the objective of theembodiment illustrated in FIGS. 12 through 16 to increase the inductanceper unit length (L′ in units of uH/m). Inductance per unit length in therange of 0.6 uH/m is desirable. This second principal embodiment canachieve this goal.

In this second principal embodiment, a reduction in the size of the rail14,6 is achieved by eliminating the interior cooling channel 5, andmoving the rail 14,6 to the outer surface of the steel barrel section13. Cooling of the rails 14,6 can be achieved by use of a water (orother evaporative fluid) spray directed to the outer surfaces of therails 14,6 after each shot. As shown in FIG. 12, this fluid spray canflow through a plurality of nozzles 9 fabricated on at least one insidesurface of a barrel section 13, above and/or below the rail 14,6. Thenozzles 9, part of the thermal management system, carry water or othercoolant for the rails 14,6 and are usually pointed in the direction ofthe rails 14,6.

FIG. 12 shows an isometric view of the second principal embodiment. Thespacing between the rails 14,6 is 30 cm for the particular design usedto calculate results discussed here, including the inductance (L′=0.55uH/m), and the magnetic computational outputs presented below. Theresults presented are not optimized at the system level. For example,with all else being held constant, if the gun bore 11 is increased from30 cm to 40 cm, L′ increases from 0.55 uH/m to 0.63 uH/m.

This second principal embodiment is also a two-rail 14,6 system. Currentis carried along the first rail 14,6 conducted across the movingarmature 30, and then returned in the opposite direction along thesecond, parallel, and opposing rail 14,6. Each rail 14 typicallyconsists of a single part made of copper, copper alloy, tungsten copperalloy, tungsten copper eutectic, or a similar wear-resistant but highlyelectrically conductive material. Alternatively, a wear-resistant cap 6can be fabricated onto primary rail part 14, as shown in FIG. 12. Cap 6is typically fabricated of steel, tungsten alloy, tungsten coppereutectic, or tungsten copper alloy.

Because of the design simplicity in this embodiment, the rail 14,6 canbe made to be removable from the barrel 13 to facilitate easyreplacement of the rail 14,6.

Shown in FIG. 13 is a detailed view of a gun barrel section 13 and itsattached current carrying rail 14,6. Typical dimensions are 52 cm by 50cm for the cross-section of barrel section 13, and 8 cm for thethickness of insulator 18. These dimensions are self-consistent with theinductance calculation results noted above.

The region between each current carrying rail 14,6 and its associatedbarrel section 13 is filled with a dielectric 18. Kevlar is thedielectric 18 of choice, though Phenolic, ceramic, or a ceramiccomposite can be used. The geometry is designed such that there is nodirect line of sight between the insulator 18/air interface and thesliding contact region between the rail 14,6 and the armature 30. Thisgeometry advantageously prevents direct UV illumination of thesesurfaces. It also prevents direct liquid metal (emanating from thesliding contact 14,6,30) or other direct sputtering or evaporativeinduced coating onto the insulator 18 surface. Additional baffles can beadded to further protect the insulator 18 if required.

Each rail 14,6 is convex in shape from the point of view of the bore 11.This allows for mechanically secure capture and guidance of the armature30 and any payload 31 along the gun bore 11. The top and bottom portionsof each rail part 14 are made to be vertical, to redirect the jettedliquid metal from the sliding rail 14,6/armature 30 interface away fromthe insulator 18 region and directly out of the gun bore 11.

FIG. 14 shows a partial assembly of the barrel 13 at a region other thanthe base 23 and turret 20. This includes the current carrying rails14,6, barrel sections 13, and now the retention frames 32 that mostlyencircle the rail 14,6 and barrel 13 assemblies. FIG. 14,6 shows two ofthe retention frames 32 that hold the two barrel sections 13 in placeduring a shot. All inward facing surfaces of the retention frames 32that would otherwise come in contact with the barrel sections 13 arelined with Kevlar or other suitable dielectric 35. This is to preventthe electrical interconnection of the two barrel sections 13 with eachother at all but one region along the length of the railgun 1. (For theembodiment where the barrel sections 13 are floating, there is no regionwhere the barrel sections 13 are electrically interconnected.) Thesedielectric sections 35, like dielectrics 18, are always undercompression.

The pitch of the retention frames 32 (distance between adjacent frames32) is large compared to the thickness of each frame 32. This is done soas not to reduce the inductance appreciably. This is also aweight-saving feature and is made possible for two reasons. First, theframe 32 height can be increased as necessary to insure that the inducedstress in the frame 32 due to the rail 14,6 current-induced magneticpressure is well within the stress limit of the steel or other strongmaterial that frame 32 is made of. Second, the barrel sections 13 aresubstantial in physical size, and themselves help to prevent outwarddeflection of the rails 14,6 in zones between each pair of retentionframes 32.

Magnetic Analysis

FIG. 15 shows a two-dimensional magnetic field line plot in which thetwo barrel sections 13 are electrically connected to each other at thegun base 23 only. The two barrel sections 13 remain electricallyisolated from each other everywhere else along the gun barrel 13 length,from the breech 21 to the muzzle 22. Physically, this means that the twobarrel sections 13 are mechanically secured to the base 23 with directmetal-to-metal connections. The retention frames 32 were not included inthe computer run upon which FIG. 15 is based. Their inclusion wouldalter the results by approximately 2% to 3%. This field line plot wastaken 100 micro-seconds into the pulse. Copper was used to simulatesteel as the material for the barrel sections 13. This simulated thecopper linings 16 that are typically used around the barrel 13 walls.

FIG. 16 shows an enlargement of the space around one of the currentcarrying copper rails 14. The field line density has been increased toshow more precisely where the surface currents flow. Drive currents flowon surfaces of the copper rail 14, and image currents flow on surfacesof the barrel 13. Once the armature 30 has passed, a percentage of therail 14 current is drawn onto the back rail surface 15 and into the gap28 between the rail 14 and the barrel section 13. By proper design, therail 14 current is distributed uniformly over the entire surface area ofthe rail 14. This advantageously minimizes energy dissipation in therail 14.

Each separatrix 27 shown in FIG. 16 denotes the location on the barrel13 surface where the surface current changes direction. These currentsreconnect at the base 23 region, where the rail 14 currents originate,and in the vicinity of the armature 30. Energy flow into the combinedcopper rail 14 and copper lining 16 can be relatively low, as comparedwith existing railgun designs. In addition, this design allows formultiple firings with minimal degradation of the rails 14,6, as therails 14,6 can be actively cooled using fluid jet techniques.

The above description is included to illustrate the operation of thepreferred embodiments, and is not meant to limit the scope of theinvention. The scope of the invention is to be limited only by thefollowing claims. From the above discussion, many variations will beapparent to one skilled in the art that would yet be encompassed by thespirit and scope of the present invention. For example, in the twoprincipal embodiments included in the above description, the barrelsections 13 had a square or non-square rectangular cross-section.However, the barrel sections can have any number of cross-sections 13,including but not limited to triangular, circular, elliptical, ortrapezoidal. Similarly, the cross-sections of the current carrying rails14 are not limited to any specific shapes or sizes.

What is claimed is:
 1. An elongated electromagnetic railgun adapted topropel a moving armature through a bore along the length of the railgunfrom its breech end to its muzzle end, said railgun comprising: twoelongated mechanically rigid electrically conductive barrel sections,said sections being spaced apart from each other along the length of therailgun; and mechanically coupled via a dielectric to each barrelsection and electrically insulated therefrom, an elongated currentcarrying rail for providing electromagnetic propulsive force to thearmature, said two rails facing each other across an elongated openchannel defining the bore; wherein: the two barrel sections areelectrically connected to each other at a maximum of one location of therailgun, said barrel sections not providing any positive electromagneticforce to the armature.
 2. The railgun of claim 1 wherein the two barrelsections are substantially identical to each other, and the two railsare substantially identical to each other.
 3. The railgun of claim 1wherein the barrel sections are fabricated of steel.
 4. The railgun ofclaim 1 further comprising an elongated dielectric shell surrounding andspaced apart from the barrel sections and rails, said dielectric shelladapted to provide containment for an inert gas.
 5. The railgun of claim4 wherein the inert gas is from the group consisting of helium,nitrogen, and argon.
 6. The railgun of claim 1 wherein the barrelsections are uniformly spaced apart from each other throughout thelength of each railgun.
 7. The railgun of claim 1 wherein the rails arefabricated of a material from the group consisting of copper, copperalloy, tungsten copper eutectic, and tungsten copper alloy.
 8. Therailgun of claim 1 wherein each barrel section has a generallyrectangular cross section.
 9. The railgun of claim 1 further comprisingan elongated electrically conductive wear-resistant plate fabricated ona bore-facing outer surface of each rail.
 10. The railgun of claim 9wherein the plates are fabricated of a material from the groupconsisting of steel, tungsten alloy, tungsten copper eutectic, andtungsten copper alloy.
 11. The railgun of claim 9 wherein severalexpansion slots are cut in each plate along the length of said plate,said slots adapted to compensate for different thermal expansioncoefficients of the plate and its corresponding rail.
 12. The railgun ofclaim 1 wherein each rail is recessed within its corresponding barrelsection.
 13. The railgun of claim 1 wherein each rail comprises anelongated internal channel adapted to deliver coolant to interiorsurfaces of the rail.
 14. The railgun of claim 13 wherein each coolantchannel is segmented along the length of its corresponding rail intoregions, permitting cooling of the rail on a region-by-region basis. 15.The railgun of claim 13 further comprising an elongated mechanicallywear-resistant plate attached to an outer surface of each rail, whereinsaid plate has a concave shape with respect to the bore.
 16. The railgunof claim 1 further comprising an electrically insulative materialpositioned between each rail and its corresponding barrel section. 17.The railgun of claim 16 wherein the electrically insulative material isfabricated of a material from the group consisting of Kevlar, ceramic,ceramic composite, and Phenolic.
 18. The railgun of claim 1 whereinouter edges of the insulation material are open to ambient gases, andthere is no direct line of sight between the electrically insulativematerial/ambient gas interface and the rail/armature interface.
 19. Therailgun of claim 1 further comprising electrically conductive linings onouter surfaces of the barrel sections, said linings fabricated of amaterial from the group consisting of copper and copper alloy.
 20. Therailgun of claim 1 further comprising several rigid retention framespositioned around the two barrel sections, said retention frames spacedapart along the length of the railgun.
 21. The railgun of claim 20wherein the retention frames are fabricated of steel.
 22. The railgun ofclaim 20 further comprising a dielectric positioned between innersurfaces of the retention frames and outer surfaces of the barrelsections.
 23. The railgun of claim 20 wherein inner surfaces of theretention frames that are positioned proximate the bore are beveled intosharp edges.
 24. The railgun of claim 20 wherein the distance betweenadjacent retention frames is large compared to the thickness of eachframe.
 25. The railgun of claim 20 wherein a slot is cut completelythrough each retention frame.
 26. The railgun of claim 1 wherein eachrail has a generally triangular cross section, with an apex of thetriangle facing towards the bore.
 27. The railgun of claim 26 furthercomprising an electrically insulated pad coupled between a portion ofeach rail and its corresponding barrel section.
 28. The railgun of claim27 wherein the insulative pad is fabricated of a material from the groupconsisting of Kevlar, ceramic, ceramic composite, and Phenolic.
 29. Therailgun of claim 26 further comprising an electrically conductive liningon outer surfaces of the barrel sections, wherein the lining is thickeron surfaces of the barrel sections that have associated rails than onsurfaces of the barrel sections that do not have associated rails. 30.The railgun of claim 26 wherein surfaces of the rails facing the boreare capped with an electrically conductive mechanically wear-resistantmaterial.
 31. The railgun of claim 30 wherein the wear-resistantmaterial is from the group consisting of steel, tungsten alloy, tungstencopper eutectic, and tungsten copper alloy.
 32. The railgun of claim 26further comprising a plurality of cooling nozzles positioned on at leastone outer surface of the barrel sections.
 33. An elongatedelectromagnetic railgun adapted to propel a moving armature through abore along the length of the railgun from its breech end to its muzzleend, said railgun comprising: two elongated mechanically rigidelectrically conductive barrel sections, said sections being spacedapart from each other along the length of the railgun; and mechanicallycoupled via a dielectric to each barrel section and electricallyinsulated therefrom, an elongated current carrying rail for providingelectromagnetic propulsive force to the armature, said two rails facingeach other across an elongated open channel defining the bore; wherein:the two barrel sections are electrically connected to each other at amaximum of one location of the railgun, said barrel sections notproviding any positive electromagnetic force to the armature; each railis recessed within its corresponding barrel section; and each railcomprises an elongated internal channel adapted to deliver coolant tointerior surfaces of the rail.
 34. An elongated electromagnetic railgunadapted to propel a moving armature through a bore along the length ofthe railgun from its breech end to its muzzle end, said railguncomprising: two elongated mechanically rigid electrically conductivebarrel sections, said sections being spaced apart from each other alongthe length of the railgun; and mechanically coupled via a dieletric toeach barrel section, an elongated current carrying rail for providingelectromagnetic propulsive force to the armature, said two rails facingeach other across an elongated open channel defining the bore; wherein:the two barrel sections are electrically connected to each other at amaximum of one location of the railgun; each rail has a generallytriangular cross-section, with an apex of the triangle facing towardsthe bore; and surfaces of the rails facing the bore are capped with anelectrically conductive mechanically wear-resistant material.